1. Field of the Invention
This invention relates to insulated flexible pipelines, pipeline insulating materials which utilize a bituminous component and particularly to a more cost-effective, insulated offshore pipeline.
2. Related Art
At low temperatures, the flow through pipelines can be impeded by high viscosity and wax formation in liquid products such as oil, and by hydrate formation in products such as natural gas. These problems can be reduced by using thermally insulated pipelines, but insulated pipelines are expensive on land and even more costly offshore. For offshore pipelines it has usually been more cost-effective to reduce the need for insulation by injecting various chemicals into the product. Recently, however, more and more oil and gas is being produced in deeper, colder water, from sub-sea production systems where use of viscosity reducing chemicals requires a dedicated line to transport them to the wellhead. This, combined with the fact that the cost of insulating pipelines typically increases with depth, means that insulated pipelines are most expensive where the alternatives are least attractive.
Various materials have been used to insulate land pipelines, including expanded cork, polymer foams, calcium silicate and others. Insulating pipelines offshore is somewhat more complicated because most insulating materials can become saturated in water when submerged. Some insulating materials incorporate watertight closed-cell structures, but all have some depth limit at which the cellular structure will collapse, and most will fail in a few hundred feet of water. Furthermore, most common insulating materials have little resistance to impact, abrasion or crushing, and must therefore be encased. If the water depth exceeds the hydrostatic pressure limitations of the material then the casing must also isolate the insulating material from the hydrostatic head of the water.
If the pipeline is laid in sections it is a practical necessity to prefabricate each individual pipe section as an independent pressure vessel. Because pressure resistant double pipes are too stiff to spool, several reel laid pipelines have been installed with flexible coatings of solid, elastomers or elastomers filled and extended with other lightweight materials. Examples include neoprene and EPDM rubber, EPDM and polyurethane elastomers filled with glass micro-spheres, and ebonite filled with cork. Unless the insulation requirement is minimal, the total cost of pipelines insulated in this manner is even higher than one which uses a pressure resistant casing to protect less expensive insulating materials.
Partly because of this expense, pipeline contractors are increasingly using "the controlled depth tow method" for laying bundles of several small pipelines that carry oil from underwater wellheads to nearby platforms. In this method, several small pipelines are fabricated onshore, inside a larger casing pipe as illustrated in FIG. 8. The casing pipe, 33, is sized to act as a floatation for the pipelines, 32. Techniques for changing ballast are used to keep the overall density of the bundle very near to the density of water so that it can be suspended between tugs at either end while it is towed to its offshore site. Once it is arrives at the destination, ballast is added to cause it to sink to the sea bed. The ballast must be heavy enough to cause the bundle to be stable on the sea bed in the presence of prevailing water currents. Pipelines have been similarly towed along the bottom, but here too, weight is closely controlled to reduce drag. If insulation is necessary or desirable, then it is desirable that the ballast have low thermal conductivity. In the past, gelled petroleum products have been used. A slurry of bentonite, water and fly ash cenospheres has also been used.
The thermal resistance offered by paint and corrosion coatings is slight due to the fact that the corrosion coatings are generally thin. Bituminous coatings were once commonly used for corrosion coatings for offshore pipelines. "Coat and wrap" coatings comprise two or three layers of Kraft paper, felt or fiberglass fabric, that are wrapped onto the pipe as they are being impregnated with hot asphalt or coal tar bitumen that is extended with finely divided mineral fillers such as fly ash, talc, finely divided silica or calcium carbonate. These coatings are 0.90 to 0.250 inches thick. "Pipeline mastic" coatings are thicker layers of asphalt concrete extruded onto the pipe. Pipeline mastic comprises calcium carbonate, sand, gravel, fiber glass and asphalt, and is 1/2 to 3/4 inch thick. In both types of coatings the fillers reduce cost and build viscosity, but the effective thermal conductivity of these fillers is five to ten times that of pure bitumen, and they therefore substantially increase the conductivity of the composition. The thermal conductivity of mastic coatings, for example is 3.5 to 4 times that of pure bitumen. The thermal conductivity of "coat and wrap" coatings is somewhat lower depending on the fabric wrap, but they are so thin that they provide less insulating value than the naturally occurring phenomenon of "self burial" of the pipeline due to scour and currents. Because the fabric layers are separated by very thin layers of bitumen, porous fabrics such as felt will eventually absorb water.
Terminology that is used to describe the most common types of bitumen is used inconsistently. "Asphalt" originally applied to natural deposits of petroleum bitumen and sand. Most asphalt used today is made from refined petroleum bitumen, which is essentially pure hydrocarbon, It was originally called "straight run asphalt", but this is sometimes shortened to "asphalt". In almost all commercial applications mineral fillers are reintroduced to reduce cost and tackiness and to increase dimensional stability at warm temperature. Thus, the word asphalt can refer to pure petroleum bitumen, or to bitumen and other earthy matter. Because pure bitumen is normally only used as a component in other asphalt products, its thermal conductivity is not tabulated in most general engineering handbooks. Values for these products vary widely, but are commonly listed simply as asphalt. Many oil exploration and production industry handbooks, for example, publish thermal conductivity of asphalt as it is found in oil wells, comprising hydrocarbons and other minerals. Pipeline industry reports give thermal conductivity for asphalt and coal tar pipe coatings. In fact, the thermal conductivity of pure bitumen is among the lowest of all solid materials. Various polymeric modifiers are also known to extend the range of temperatures over which bitumen remains dimensionally stabile. Some do so by chemically reacting with, or in the presence of the bitumen while others behave as a second continuous phase that forms a network, or has an affinity with the bituminous component. U.S. Pat. No. 5,306,750, for example, disclosed that epoxide containing polymers can be caused to react to improve resistance to deformation. Some of those that have been shown to improve dimensional stability by forming a continuous phase network with the bitumen include styrene butadiene (SBR) rubber styrene-butadiene-styrene (SBS) rubber, styrene-ethylene/butylene-styrene (SEBS) rubber, ethylene vinyl acetate, other block co-polymers, polyolefins, neoprene latex, and other elastomeric materials. Some of these can lower the of embrittlement temperature and raise the melting temperature. The effect of fillers is to raise viscosity at all temperatures, even above the melting point of the bitumen. Highly filled bitumen behaves as a mastic, even when the bitumen itself is a molten liquid. The combination of fillers and polymeric modifiers can increase softening point beyond what fillers alone or polymeric modifiers alone can achieve.
Some polymeric modifiers are finely divided particles such as crumb rubber or polyolefins. These materials may behave more like fillers than as a second continuous phase and unlike mineral fillers, their elastomeric properties also increase the flexibility of asphalt at low temperature Some of these polymers melt, but behave as an emulsion rather than a continuous phase when mixed with molten bitumen. Mineral fillers are usually used to reduce cost and increase dimensional stability of bitumen whether or not it contains chemical or polymeric modifiers that react or form a network with the bitumen.
Sulfur and carbon black are also known to increase the softening point of bitumen,